The friction coupling can be used in a differential assembly for variably distributing torque between two output shafts or as a hang-on coupling for an optionally drivable driving axle of a motor vehicle. A differential assembly featuring a variable torque distribution is known for example from U.S. Publication No. 2006/0172845, whereas U.S. Pat. No. 5,423,235 describes an example of a hang-on coupling.
Said friction couplings serve control purposes, more particularly they control the distribution of torque at two wheels of a driven axle or the distribution of torque between two drivable axles. To be able to carry out said control procedures, the coupling torque transmitted by the coupling has to be known and has to be continuously determined by suitable means. Because of changes in the friction coefficient of the friction lining which occur as a function of temperature and service life, determining the coupling moment via the pressure force of the friction plates is an inaccurate approximation of the actual coupling moment. In sophisticated lock and vectoring concepts with a variable torque distribution, inaccurate coupling moment estimates are inadequate and lead to complicated control methods or a limited use of the systems.
WO 2005/064209 A1 proposes a method of determining the coupling moment as well as a friction coupling with an electromechanical actuator. The actuator comprises a supporting disc axially fixed in a housing and a displaceable setting disc axially supported on the supporting disc. The supporting disc is provided in the form of an annular piston which is arranged in an annular chamber filled with a hydraulic fluid. For measuring the hydraulic pressure, a pressure sensing element is arranged in the annular chamber in the housing. The measured pressure is converted into an estimate of the transmitted coupling moment.
Accordingly, there exists a need for improved systems for accurately determining a torque at a friction coupling.